Method of forming a cooling device for an integrated circuit

ABSTRACT

A pump having: a cavity formed inside an insulating substrate, the upper part of the substrate being situated near the cavity having an edge; a conductive layer covering the inside of the cavity up to the edge and optionally covering the edge itself; a flexible membrane made of a conductive material placed above the cavity and resting against the edge; a dielectric layer covering the conductive layer or the membrane whereby insulating the portions of the conductive layer and of the membrane that are near one another; at least one aeration line formed in the insulating substrate that opens into the cavity via an opening in the conductive layer, and; terminals for applying a voltage between the conductive layer and the membrane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.10/580,324, filed on May 18, 2007, now U.S. Pat. No. 8,164,183 whichapplication is a national stage of International Application No.PCT/FR2004/050584, filed on Nov. 12, 2004, which application claims thepriority benefit of French Patent Application No. 03/50910, filed onNov. 25, 2003, which applications are hereby incorporated by referenceto the maximum extent allowable by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to micropumps and in particular to theiruse in an integrated circuit cooling device.

2. Discussion of the Related Art

A known cooling device is a metal heat sink placed against a surface ofan integrated circuit chip. The carrying off of the heat from a “hot”area of the circuit to the heat sink is performed through a portion ofthe circuit generally exhibiting poor thermal conductivity.

As a complement to such a heat sink, or if no other cooling device ispresent, the integrated circuit is placed in an enclosure comprising ablower generating a permanent air current around the circuit.

These two cooling devices, associated or not, may not provide sufficientcooling down of a circuit having a high active density of components.

An object of the present invention is to provide a cooling devicecapable of maintaining at an acceptable level the temperature of anintegrated circuit comprising a large number of active components.

A more general object of the present invention is to provide amicropump.

To achieve this, the present invention provides a pump comprising: acavity formed in an insulating substrate, the upper portion of thesubstrate located in the vicinity of the cavity forming a border, aconductive layer covering the inside of the cavity all the way to theborder and possibly covering the border, a flexible membrane, formed ofa conductive material, placed above the cavity and bearing against theborder, a dielectric layer covering the conductive layer or the membraneto insulate the portions of the conductive layer and of the membranewhich are close to each other, at least one ventilating duct formed inthe insulating substrate which emerges into the cavity through anopening of the conductive layer, and terminals of application of avoltage between the conductive layer and the membrane.

According to an embodiment of the above-mentioned pump, said cavity hassubstantially the shape of a cup such that the interval between theconductive layer and the membrane progressively increases from theborder to the bottom of the cavity.

According to an embodiment of the above-mentioned pump, the membrane isin an idle state when no voltage is applied between said terminals, theapplication of a voltage deforming the membrane by drawing it closer tothe conductive layer, the removal of the voltage bringing the membraneback to its idle state.

According to an embodiment of the above-mentioned pump, the pumpcomprises a single ventilating duct emerging substantially at the bottomof the cavity.

According to an embodiment of the above-mentioned pump, the pumpcomprises two ventilating ducts, one emerging substantially at thebottom of the cavity, the other one emerging close to the border.

According to an embodiment of the above-mentioned pump, the pump isconnected to an assembly of ventilating ducts formed in thesemiconductor substrate of the integrated circuit.

The present invention also provides a method for forming a pump in anintegrated circuit, comprising the steps of: forming a cavity in aninsulating substrate, the upper portion of the substrate located in thevicinity of the cavity forming a border; covering the inside of thecavity all the way to the border and possibly the border with a firstconductive layer; forming an opening of the conductive layer emerginginto a ventilating duct previously formed in the insulating substrate;filling the cavity with a sacrificial portion; covering the sacrificialportion and the portion of the first conductive layer placed above theborder with a first insulating layer and with a second insulating layer;forming a small opening in the second conductive layer and in the firstinsulating layer; removing the sacrificial portion; and covering thesecond conductive layer with a second insulating layer to close back theopening.

According to an embodiment of the present invention, the step of forminga cavity in an insulating substrate comprises the steps of: forminginsulating pads on a first insulating layer; covering the firstinsulating layer and the insulating pads with a second insulating layer;and performing a chem.-mech. polishing of the second insulating layer toexpose the insulating pads, the etch method of the polishing being suchthat it etches the second insulating layer more than the insulatingpads, the insulating pads forming said border.

The present invention also provides a method for actuating a pump suchas described hereabove, in which a voltage is applied at regular orirregular intervals between said terminals.

The foregoing object, features, and advantages of the present inventionwill be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are cross-section views of a pump according to the presentinvention in two operating states;

FIG. 3 is a top view of the pump shown in FIGS. 1 and 2;

FIGS. 4 and 5 are cross-section views of another example of a pumpaccording to the present invention in two operating states;

FIGS. 6A to 6I are cross-section views of structures obtained insuccessive steps of a method for forming a pump according to the presentinvention; and

FIG. 7 is a cross-section view of an example of an integrated circuitcomprising a pump according to the present invention.

DETAILED DESCRIPTION

As usual in the representation of integrated circuits, the variousdrawings are not to scale.

1. Pump

FIGS. 1 and 2 are cross-section views of a pump according to the presentinvention respectively in an idle state and in an activation state. FIG.3 is a top view of the pump shown in FIGS. 1 and 2. The pump is formedabove an insulating substrate 1 and more specifically in an upper cavity2 of substrate 1. Cavity 2 has in this example the shape of a cup. Theupper portion of substrate 1 located in the vicinity of the cavity formsa border, having in this example a circular shape such as visible inFIG. 3. The inside and the border of cavity 2 are covered with aconductive layer 3, for example, made of copper or of aluminum. Anopening O1 of conductive layer 3 is formed substantially at the bottomof cavity 2 above a ventilating duct 4 formed in substrate 1.Ventilating duct 4 emerges outside of the substrate. A flexible membrane6, formed of a conductive material, is placed above cavity 2 and bearsagainst the border of cavity 2 above conductive layer 3. Membrane 6 andconductive layer 3 are insulated from each other by an insulating layer7 covering in this example the lower surface of flexible membrane 6.Conductive layer 3 and flexible membrane 6 are connected to twoterminals between which a control circuit V applies a voltage whenordered to do so.

In the idle state, when control circuit V applies no voltage, membrane 6and insulating layer 7 are substantially horizontal, as shown in FIG. 1.In the activation state, when control circuit V applies a voltage,membrane 6 deforms by coming closer to conductive layer 3, as shown inFIG. 2. When membrane 6 deforms, the volume of the air pocket locatedbetween membrane 6 and conductive layer 3 decreases, which results inchasing the air towards ventilating duct 4. When control circuit V stopsapplying a voltage, membrane 6 separates from conductive layer 3 toreturn to its idle state horizontal position. The volume of the airpocket then progressively increases, which results in letting air intoventilating duct 4. By successively repeating the operations ofdeformation and release of membrane 6, it is thus possible toalternately let in “fresh” air and let out “hot” air.

According to an alternative embodiment of the above-described pump,insulating layer 7 covers conductive layer 3. Opening O1 is then formedthrough insulating layer 7 and conductive layer 3.

FIGS. 4 and 5 are cross-section views of another example of a pumpaccording to the present invention respectively in an idle state and inan activation state. The pump has a structure substantially identical tothat of the pump shown in FIGS. 1 to 3. The pump further comprises asecond ventilating duct 10 connected to a second opening of thesubstrate and emerging into a second opening O2 of conductive layer 3formed close to the border of cup-shaped cavity 2.

When control circuit V applies a voltage, membrane 6 progressivelydeforms and by coming closer to conductive layer 3, it covers openingO2. Then, the increasing deformation of the membrane reduces the airpocket volume and chases hot air out of ventilating duct 4. When controlcircuit V stops applying a voltage, membrane 6 progressively relaxes toreturn to its idle state. As long as membrane 6 covers opening O2, airenters the cavity through ventilating duct 4. As soon as opening O2 isuncovered, air enters cavity 2 through the two ventilating ducts 4 and10.

In the case where the size of opening O2 is much larger than that ofopening O1, the air volume entering through opening O2 is much largerthan that entering through opening O1. Thus, upon relaxation of membrane6, it is possible to fill cavity 2 with air principally coming fromventilating duct 10. Accordingly, the coming in of “fresh” air intocavity 2 mainly occurs through ventilating duct 10 and the coming out of“hot” air mainly occurs through ventilating duct 4.

As an example, the sizes of the various pump elements are the following:

-   -   diameter of the cup-shaped cavity: from 100 to 1000 μm    -   maximum depth of the cavity (at the center): 15 μm    -   diameter of ventilating duct 4 (opening O1): from 1 to 10 μm    -   diameter of ventilating duct 10: from 1 to 10 μm    -   thickness of conductive layer 3: from 100 nm to 2 μm    -   thickness of membrane 6: 2 μm

When the control circuit applies a voltage between layer 3 and membrane6, the deformation of membrane 6 is progressive. The portions of layer 3and of membrane 6 located in the vicinity of the border of cavity 2 areclose to each other and a small voltage enables drawing them closer.Once these first portions have been drawn closer to each other, theportions of layer 3 and of membrane 6 located right next to them arethen close to each other and a small voltage enables drawing themcloser, and so on. The maximum deformation is that for which the“mechanical” membrane restoring force becomes equal to the electrostaticforce created between layer 3 and membrane 6 by application of a voltageby control circuit V.

An advantage of the above-described pumps is that they can be activatedwith a small voltage.

The above-described pumps have a cup shape, which has theabove-mentioned advantage. However, other cavity shapes may be imaginedin which the conductive layer placed inside of the cavity and theflexible membrane placed above the cavity are not necessarily in contacton the cavity border.

2. Pump Manufacturing Method

A pump according to the present invention can be formed according to themethod described hereafter.

In an initial step, illustrated in FIG. 6A, a cup-shaped cavity 20 isformed in an insulating substrate 21. The upper part of the substratelocated close to the cavity forms a border. The cavity will preferablybe “cup”-shaped so that the cavity depth progressively increases fromthe border to the bottom of the cavity.

The cup shape can be obtained according to the following method.Insulating pads 23 and 24 are formed on an insulating layer 22.Insulating layer 22 and possibly pads 23 and 24 are then covered with asecond insulating layer 25. A chem.-mech. polishing of second insulatinglayer 25 is then performed to expose insulating pads 23 and 24. The etchmethod implemented in the polishing is selected to that it “etches”insulating layer 25 more than pads 23 and 24. When pads 23 and 24 arerelatively spaced apart, a recess forms in insulating layer 25 betweenpads 23 and 24. This phenomenon, known as “dishing”, is generally notdesirable since it results in the forming of non-planar surfaces.However, advantage is taken of this phenomenon in the method of thepresent invention to form a cup-shaped cavity.

At the next step, illustrated in FIG. 6B, the inside and the border ofcavity 20 are covered with a conductive layer 30, for example, made ofaluminum.

At the next step, illustrated in FIG. 6C, conductive layer 30 is etchedto form an opening O3 at the bottom of cavity 20 above a ventilatingduct 31 previously formed in substrate 21.

At the next step, illustrated in FIG. 6D, cavity 20 is filled with asacrificial portion 32. Sacrificial portion 32 does not cover the borderof cavity 20. A method of sacrificial layer deposition which conforms aslittle as possible may be used to avoid filling ventilating duct 31. Anetch or a chem.-mech. polishing of the sacrificial layer is thenperformed to remove the portions covering the border of cavity 20.

At the next step, illustrated in FIG. 6E, an insulating layer 33 isformed above sacrificial portion 32 and above the portions of conductivelayer 30 located on the border of cavity 20.

At the next step, illustrated in FIG. 6F, insulating layer 33 is coveredwith a conductive layer 34.

At the next step, illustrated in FIG. 6G, a small opening O4 is formedin conductive layer 34 and in insulating layer 33 to reach underlyingsacrificial portion 32.

At the next step, illustrated in FIG. 6H, sacrificial portion 32 isremoved through opening O4, for example, by etching.

At the next step, illustrated in FIG. 6I, conductive layer 34 is coveredwith a thin insulating layer 35 according to a method which is as littleconformal as possible so that the deposited insulating layer penetratesas little as possible through opening O4.

3. Pump Placed in an Integrated Circuit

A pump according to the present invention may be used to have air oranother fluid flow through an assembly of ventilating ducts formed in anintegrated circuit to cool it down. An example of ventilating ducts anda method for forming such ventilating ducts are described in“Micromachining of Buried Micro Channels in Silicon”, JOURNAL OFMICROELECTROMECHANICAL SYSTEMS, Vol. 9, No 1, March 2000, which isincorporated herein by reference.

FIG. 7 is a cross-section view of an example of an integrated circuitcomprising a pump according to the present invention. Components 40,such as MOS transistors, are formed at the surface of a semiconductorsubstrate 41. A network of ventilating ducts 42 is provided insemiconductor substrate 41. A network of metal interconnects 43 isplaced above components 40 and substrate 41. Interconnect network 43comprises in this example five metallization levels on which are formedvarious conductive lines. Conductive vias enable connecting conductivelines placed on two adjacent levels. A micropump according to thepresent invention is placed in this example above interconnect network43 and more specifically in a cup-shaped cavity 45 formed in the upperinsulating layer of the last metallization level. A conductive layer 46covers the inside and the border of cavity 45. A conductive layer 47,covered at its lower surface with an insulating layer 48, is placedabove cavity 45 by bearing against the border. A vertical opening,corresponding to a duct 49, is formed through interconnect network 43.Duct 49 emerges on the one hand into cavity 45 of the pump through anopening of conductive layer 46 and on the other hand into ventilatingduct 42 provided in semiconductor substrate 41. The pump is placed undera protection “bell” formed of an insulating portion 54 substantiallyhaving the shape of a hemisphere laid on interconnect network 43.

On one of the sides of cavity 45, insulating layer 48 extends topartially cover the upper insulating layer of interconnect network 43.Conductive layer 47 continues above the extension of insulating layer 48to cover a portion of the upper insulating layer in which is placed aconductive via 50 connected to a conductive line 51 of interconnectnetwork 43. Conductive layer 46 is connected to a conductive line 52 ofthe interconnect network via a conductive via 53 placed under conductivelayer 46. Conductive lines 51 and 52 enable connecting conductive layers46 and 47 to a control circuit V formed in the integrated circuitsubstrate.

Such an integrated circuit may comprise a temperature sensor. Thecontrol circuit may activate more or less rapidly the pump according tothe measured temperature.

Other embodiments of an integrated circuit comprising a pump accordingto the present invention may be imagined. The pump may for example beplaced right above semiconductor substrate 41 under interconnect network43.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, those skilled in the art may imagineother methods for manufacturing a pump according to the presentinvention. Further, the number and the location of the openings formedin the lower conductive layer of the pump will be determined accordingto the ventilating ducts provided in the integrated circuit.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the scope ofthe invention. Accordingly, the foregoing description is by way ofexample only and is not intended as limiting. The invention is limitedonly as defined in the following claims and the equivalents thereto.

What is claimed is:
 1. A method for forming a pump in an integratedcircuit, the method comprising: forming a cavity in a first insulatinglayer, the upper portion of the first insulating layer located in thevicinity of the cavity forming a border; covering the inside of thecavity all the way to the border and possibly the border with a firstconductive layer; forming a first opening of the first conductive layeremerging into a ventilating duct previously formed in the firstinsulating layer; filling the cavity with a sacrificial portion;covering the sacrificial portion and the portion of the first conductivelayer placed above the border with a second insulating layer and with asecond conductive layer; forming a small second opening in the secondconductive layer and in the second insulating layer; removing thesacrificial portion; and covering the second conductive layer with athird insulating layer to close back the small second opening.
 2. Themethod of claim 1, wherein the step of forming a cavity in a firstinsulating layer comprises the steps of: forming insulating pads on afourth insulating layer; covering the fourth insulating layer and theinsulating pads with a fifth insulating layer; and performing a chemicaland mechanical polishing of the fifth insulating layer to expose theinsulating pads, the etch method of the polishing being such that itetches the fifth insulating layer more than the insulating pads, theinsulating pads forming said border.
 3. A method for actuating a pump ofan integrated circuit chip of claim 1, comprising applying a voltage atregular or irregular intervals between terminals.
 4. The method of claim1, further comprising providing a temperature sensor and a controlcircuit and activating the pump according to a desired temperature. 5.The method of claim 1, wherein forming the cavity comprises forming acavity having a cup-shape so that a depth of the cavity is progressivelyincreasing from the border to a bottom of the cavity.
 6. The method ofclaim 1, wherein forming the first conductive layer comprises formingthe first conductive layer of aluminum.
 7. The method of claim 1,wherein forming the first opening comprises positioning the firstopening substantially at a bottom of the cavity.
 8. The method of claim1, wherein forming the first opening comprises etching the firstconductive layer.
 9. The method of claim 1, wherein removing thesacrificial portion comprises etching.
 10. The method of claim 1,wherein removing the sacrificial portion comprises etching or chemicaland mechanical polishing the border of the cavity.
 11. The method ofclaim 3, wherein applying the voltage comprises applying the voltagebetween terminals located on each of the first and second conductivelayers.
 12. The method of claim 3, wherein applying the voltagecomprises causing the second conductive layer to move.